Just want to add myself to the list of people struggling with this.
I'm on an 8 core Mac Pro running the non-subscription version of Parallels 14 in Mojave 10.14.5. The experience is pretty dire.
Windows was, at first, completely unusable. And I couldn't access Parallels Configuration settings because the VM seemed to restart Windows.
Thanks to this note --
https://kb.parallels.com/124449 -- I managed, incredibly slowly, to change the Windows settings so that I could actually shut down.
I was then able to access the Parallels Configuration settings.
* Previous versions of Parallels had run happily in 4 cores. Now the 'recommended' setting is 2. I set it to that. (Subscription version may be different.)
* I also set the RAM allocation to half of what I have available -- that is, 6144 MB (I have 12 GB total).
* I set the Graphics memory to Auto.
Windows 10 now runs. It's still very slow. All controls take a long time to respond. Typing is like watching grass grow. I can't say I'm happy, but at least I can do limited web testing.
In addition, the VM's CPU Usage icon at top right is constantly red, with CPU usage showing at 100%. This can't be good.
Like others, I have found Parallels to be quite usable in the past. I do have to laugh every year at the claims of huge increases in performance. We'd be running faster than PCs by now if they were true! But this combination -- Mojave and Parallels 14 -- seems to have set us back a long way.